1. Field of the Invention:
This invention relates to a process for producing a high purity fused quartz glass with a low moisture content.
2. Description of the Prior Art:
A typical method of making fused quartz glass of a grade suitable for use as low-loss optical fibers is the chemical vapor-phase process wherein fine grains of glass (hereunder referred to as soot) generated by the use of an oxygen/hydrogen burner are deposited on a starting member. This method is more productive than other conventional methods, for example, the "internal sooting process" wherein a glass tube provided with reaction heat from an external source is supplied with gases carrying a material for glass which forms a soot that is deposited on the internal wall of the glass tube. These methods are discussed in commonly assigned U.S. Pat. No. 4,224,046. Even glasses made by chemical vapor-phase process are subject to light loss due to the presence of OH groups generally in an amount of about 20 to 50 ppm. Thus, various efforts have been made to decrease the OH group content in glasses adapted for use in low-loss optical fibers.
According to one of these methods, the soot generated by chemical vapor-phase process is deposited in a desired form and the deposit is sintered in a chlorine content atmosphere to form a vitreous substance. This method, however, is not advantageous since it is very difficult to sufficiently diffuse and penetrate chlorine into the inner portion of the glass and, in order to reduce the residual OH group content to a minimum extent, the treatment with chlorine must be conducted for a prolonged period of time. Further, a transition metal, particularly an iron (II) compound, contaminates the deposit as a result of the treatment with chlorine.
Other proposals for reducing the content of OH groups in silica glass are described in Japanese Patent Publications Nos. 42047/78 dated Nov. 8, 1978 (corresponding to U.S. Pat. No. 4,038,370) and 42335/78 dated Nov. 10, 1978. In these methods, a silane-containing gas is first oxidatively decomposed in an excess of hydrogen gas to provide a vitreous substance containing a bond having an oxygen defect. Thereafter, the vitreous substance is heat-treated such that the OH groups present between the glass molecules act on the oxygen-defect site so as to remove the OH groups as hydrogen gas. However, even these methods are unable to achieve complete removal of the OH groups because the reaction is carried out in the presence of an excess of hydrogen gas and the resulting glass contains much water (OH groups).